1. Field of the Invention
The present invention relates to a wire bonding method and apparatus that forms a ball at the tip of a bonding wire by applying a high voltage across the tip of the wire and an electrode.
2. Prior Art
Conventional electrodes used in wire bonding apparatuses are disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) No. 47-859 (called "Conventional Example 1"), Japanese Utility Model Application Publication (Kokoku) No. 57-21325 (called "Conventional Example 2"), and Japanese Patent Application Laid-Open (Kokai) No. 5-102233 (called "Conventional Example 3").
In the electrode of Conventional Example 1, the tip end of the electrode 13 has a truncated cone shape, and the flat surface at the tip of this truncated cone is used as an electric discharge part. In the electrode of Conventional Example 2, the electrode 10 has an inclined flat surface or inclined and curved surface with respect to the axial line of the wire so as to be used as an electric discharge part. In the case of the electrode 17 of Conventional Example 3, the flat upper surface of the electrode is used as an electric discharge part. The flat upper surface is covered with an insulating film except for a certain area of the flat surface.
Generally, the charge for an electrical discharge tends to concentrate at edge sections or projecting portions of electrodes. However, in all of the conventional electrodes described above, the discharge parts are flat or curved. Accordingly, there is no place for the charge to concentrate, and the force of the discharge is therefore weak. Thus, since a strong discharge cannot be obtained in a short period of time, the discharge is unstable, and the ball to be formed is also unstable. In other words, the formed balls are unstable in diameter.
FIG. 6 shows an existing conceivable approach to overcome such diameter differences or unstable balls. In this prior art, an electrode 4 is made from a round bar, and the tip end is of a sharp conical shape. In FIG. 6, reference numeral 1 indicates a bonding wire which passes through a capillary 2, and a ball 6 is formed when a high voltage from a high-voltage generator 3 is applied via the electrode 4 to the tip of the wire 1 so that a discharge 5 occurs. If the discharging part of the electrode 4 is thus formed into a sharp conical shape, the charge can concentrate at the sharp tip. Thus, it would appear that a strong discharge could be obtained in a short period of time so that balls 6 formed at the tip of the wire is consistent in diameter.
However, experiment indicates that the discharge to the wire also occurs from the side surfaces and undersurface of the tip end of the electrode. Thus, the discharge is not stable, and the balls are not in the same diameter. Furthermore, since the electrode is usually formed from an extremely hard substance, e.g., tungsten, high-precision working is extremely difficult. Accordingly, when the material is worked so as to form the end thereof into a conical shape, projections referred to by reference numeral 7 caused by irregular working may remain on the electrode. The result is that a discharge occurs from these projections 7, enabling a stable electric discharge.
Furthermore, when a strong discharge is required so as to reduce the ball diameter variations, it is necessary to limit the area of the discharge part of the electrode to as narrow an area as possible. Accordingly, the tip end of the electrode must be worked into as sharp a conical shape as possible. However, when the tip end of the electrode is formed into a sharp conical shape, the volume of the tip end is greatly reduced, and the speed at which a discharge-induced wear progresses is increased, resulting in that the useful life of the electrode is shortened.